This project involves an analysis of how infrahuman organisms (rats and pigeons) acquire, utilize, lose, and recall associations or relations between the presence or absence of one event and the presence or absence of another. The major specific topics involve (a) the feature positive effect (which refers to the apparent difficulty of many species, including man, to learn that the absence of a particular feature is the unique signal for the presence of a forthcoming event), (b) the facilitation of trace conditioning by placement of filler stimuli in the "empty intervals" of that Pavlovian arrangement, and (c) comparisons of conditioned excitation and inhibition. Signtracking (autoshaping) procedures in pigeons and the CER technique in rats provide the main methods for studying these problems. Mere presence vs. absence of stimulus elements is compared with the role of sheer predictiveness and localizability in accounting for the feature positive effect. "Perceptual" aspects of features will be examined so as to better clarify or define what a "feature" is. Sequential vs. simultaneous presentation of feature and common stimulus elements will be a focus of interest, and the findings ought to have implications for current theories of excitation and inhibition. Although feature-negative discriminations seem to be poorly learned, our recent work shows that they are clearly manifested (unmasked) in pigeons when appropriate test conditions (e.g., complete extinction) are established; the generality of the surprising outcome is to be examined and its bases analyzed. An understanding of behavioral control by the absence or removal of a stimulus is a dominant goal of planned research, as compared to control by stimulus presence or addition. All these topics will be assessed with respect to various classic and modern theories of Pavlovian vs. operant conditioning, discrimination learning, and information processing. The hope that this project will help in the analysis of brain function and evolution, and in practical settings where improvements of human memory, learning, concept formation, and reasoning are sought.